In the oil and gas industry, following drilling of a vertical or horizontal wellbore into a formation for the production of oil or gas therefrom, the wellbore is typically cased and cemented to line the length of the wellbore to ensure safe control of production of fluids therefrom, to prevent water from entering the wellbore and to keep the formation from “sloughing” or “bridging” into the wellbore.
It is well known that during the running in of a tubing string, such as casing and particularly the production casing, the casing may encounter tight spots and obstructions in the open wellbore, such as that created by sloughing of the wellbore wall into the open hole or as a result of the casing pushing debris ahead of the bottom end of the casing along the open hole until it forms a bridge. Such obstructions prevent the advance of the casing and require the open hole to be cleared in order to advance the casing to the bottom of the hole. This is particularly problematic in horizontal wellbores.
Should the casing string become sufficiently engaged in a mud pack formed at the obstruction, differential sticking may also occur, making advancing or removal of the casing from the wellbore extremely difficult.
While casing strings have been rotated to assist with moving past or through an obstruction, high torque created by trying to rotate a long string of casing may result in significant damage to the threads between casing joints and may cause centralizers and the like to drag and ream into the wellbore. While rotation of casing may be a viable option in a vertical wellbore, albeit fraught with problems, it is extremely difficult, if not impossible in a horizontal wellbore.
One option is to employ a washing technique, pumping fluids through the casing while the casing is axially reciprocated uphole and downhole. The fluids exiting the downhole end of the casing bore act on the obstruction in the wellbore to wash out or erode the wellbore obstruction creating debris which is lifted or conveyed through the annulus to surface by fluid circulation therein. Should the washing technique be unsuccessful, it is known to trip out the casing and run in a mud motor on a drill string to drill out or ream the obstruction from the wellbore. Such repeated running in and tripping out of tubulars is time consuming, labor intensive and, as a result, very expensive. Alternatively, others have contemplated providing teeth on the bottom of the casing string or on a shoe at the bottom of the casing string to assist with cutting away the obstruction as the casing is advanced during running in. Typically, the casing is also reciprocated or stroked during the clearing operation, or, in some cases, at the same time as the casing is rotated.
Further, it has been contemplated to attach costly apparatus, such as mud motors, jetting or reaming tools, to the bottom of the casing string, however the apparatus is not retrievable thereafter from the wellbore and adds significantly to the cost of the casing operation.
Ideally, what is required is a relatively simple and inexpensive apparatus that can be incorporated into the casing string for clearing wellbore obstructions without the need for rotating the casing string. Ideally, the apparatus could be left downhole, after the casing and cementing operations are complete, without a significant increase in operational costs.